This invention relates to a process and apparatus for growing aquatic animals, for example fish, in a controlled environment wherein water is continuously circulated.
A process for improving growing conditions of fish is described in DOS No. 2,110,091 wherein oxygen-enriched water, preferably pumped from a spring or a well, and an oxygen-containing gas are circulated continuously from the bottom to the top of a fish tank. A particular embodiment provides cascade flow wherein several tanks are arranged in side-by-side relationship at different heights so that overflow water from the tank located at a higher level, after purification and renewed oxygen enrichment, is passed through the tank arranged at the next-lower level. In this way, water consumption is lowered as a function of the number of cascades.
In this conventional upward-flow system, problems arise in the discharge of the metabolic wastes produced by the fish or of other waste matter from the tanks. If the upward velocity of the water is too slow, the metabolic wastes produced by the fish settle downwardly against the current of water and gas. Thus, the wastes remain suspended at the bottom of the tank or even clog up the oxygen inlet. In any case, the residence time of the fecal waste in the tank is detrimentally increased leading to such manifestations as taste impairment or poisoning of the fish.
A similar deleterious effect is produced when the upward flow of water is too rapid, owing to the resultant turbulence which suspends the fecal waste throughout the tank.
Accordingly, the amount of water conducted upwardly through the tanks is critical for optimum environmental growth conditions. However, it is not a simple task to quickly adjust and maintain the water flow within the prescribed or critical limits, and this complicates the growing operation. Furthermore, as the fish grow, the rate of oxygen introduced into the tanks must in turn be increased, and this can likewise lead to detrimental turbulent conditions.